Railway tread braking temperatures - Numerical simulation and experimental studies

Licentiate thesis, 2012

Tread (block) braking is still one of the most common braking systems on railway vehicles. The action is carried out by pressing brake blocks against the tread of a wheel, which is also in rolling contact with the rail. The extensive use of tread brakes in metro and suburban applications has created a need for design guidelines or standards for wheels exposed to repeated stop braking. The thermal capacity of the wheels puts a limit to railway tread braking systems. With the exception of the drag braking cases described in the European standard EN 13979-1, there are no known standards or guidelines regarding the thermal capacity limits for wheels. In the present work, an extensive literature survey has been made with special focus on the braking capacity of wheels. Several aspects of the tread braking system, important for the dimensioning of railway wheels, have been assessed, such as brake block materials and residual stresses and temperature gradients through wheel rim and wheel disc. Additionally, two different railway wheel designs, with typical characteristics of freight and metro wheels, have been numerically studied with respect to design criteria for load cases of drag braking and stop braking. Brake rig experiments and a field test campaign were performed and aimed at measuring wheel and brake block temperatures during different service conditions for a metro line. It was concluded that even though the same nominal routes were simulated in the brake rig tests as those the field tests, the braking efforts are different. Therefore, simulation and calibration tools were employed in order to facilitate a comparison between measured temperatures. The results showed the importance of knowing the convection cooling parameters for different wagons if prolonged braking action is to be considered. Heat partitioning between wheel, block and rail has been numerically studied in a broad parametric study to investigate the influence of brake block materials, thermal parameters and brake pressure distribution. By use of a plane model, the implication of temperature variations around the wheel circumference (hot spots) is studied in detail. Even though the hot spots have a major impact on local temperatures, they were found to have only a minor influence on the global heat partitioning in the wheel-block-rail system. By use of an axisymmetric model, it was found that a presumed constant axial position of the wheel-rail contact towards the flange side of the tread leads to substantially higher maximum tread temperatures than a wheel-rail contact centred at the brake block position.